processes Review Effect of Processing on Phenolic Composition of Olive Oil Products and Olive Mill By-Products and Possibilities for Enhancement of Sustainable Processes Fereshteh Safarzadeh Markhali CEB—Centre of Biological Engineering, Campus of Gualtar, University of Minho, 4710-057 Braga, Portugal; [email protected] or [email protected] Abstract: The bio-functional properties of olive oil products and by-products rely greatly on the proportions and types of the endogenous phenolics that may favorably/unfavorably change during various processing conditions. The olive oil industrial activities typically produce (i) olive oils, the main/marketable products, and (ii) olive mill by-products. The mechanical processing of olive oil extraction is making progress in some areas. However, the challenges inherent in the existing system, taking into consideration, the susceptibilities of phenolics and their biosynthetic variations during processing, hamper efforts to ascertain an ideal approach. The proposed innovative means, such as inclusion of emerging technologies in extraction system, show potential for sustainable development of olive oil processing. Another crucial factor, together with the technological advancements of olive oil extraction, is the valorization of olive mill by-products that are presently underused while having great potential for extended/high-value applications. A sustainable re-utilization of these Citation: Safarzadeh Markhali, F. valuable by-products helps contribute to (i) food and nutrition security and (ii) economic and Effect of Processing on Phenolic Composition of Olive Oil Products environmental sustainability. This review discusses typical processing factors responsible for the and Olive Mill By-Products and fate of endogenous phenolics in olive oil products/by-products and provides an overview of the Possibilities for Enhancement of possibilities for the sustainable processing to (i) produce phenolic-rich olive oil and (ii) optimally Sustainable Processes. Processes 2021, valorize the by-products. 9, 953. https://doi.org/10.3390/ pr9060953 Keywords: processing; phenolics; olive oil products; olive mill by-products; sustainability; extraction Academic Editors: Jorge A. Ferreira, Tobias Richards, Timo Kikas and Jorge Mario Marchetti 1. Introduction Polyphenols, the secondary metabolites and the predominant groups of phytonutri- Received: 9 May 2021 ents in plants, are highly valued for their bio-functional properties and defense mechanisms. Accepted: 26 May 2021 Among other benefits, they confer antioxidative [1] and antimicrobial activities [2,3]. Of all Published: 27 May 2021 the plant-derived foodstuff, the unique healthful and organoleptic attributes of olive oil products have noticeably caught the global attention, signifying their distinctive chemical Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in and molecular characteristics [4]. In this regard, the agro-industrial activities of olive published maps and institutional affil- crops, which have been traditionally and economically substantial particularly in the iations. Mediterranean region, have progressively increased primarily for olive oil production [5,6]. Olive oil, on top of being a good source of unsaturated fatty acids (around 72% monoun- saturated fatty acids primarily oleic acids, and 14% polyunsaturated fatty acids [7]), is highly valued as a source of minor bioactive compounds including phenolics [8,9]. The bio-functional potential of phenolics, however, relies greatly on their proportion, molecular Copyright: © 2021 by the author. structure/interactions, and chemical metabolism [10]. In the olive oil industry, there are Licensee MDPI, Basel, Switzerland. This article is an open access article different classes of olive oil products, typically characterized by the specified/standard distributed under the terms and quality parameters. The main groups, among others, include extra virgin olive oil (EVOO), conditions of the Creative Commons virgin olive oil (VOO), and refined olive oil [7]. Of all designated groups, EVOO is known Attribution (CC BY) license (https:// as the highest quality, containing total polyphenols in the range of 50 to 1000 mg/Kg, and creativecommons.org/licenses/by/ acidity level below 0.8 g/100 g [11]. 4.0/). Processes 2021, 9, 953. https://doi.org/10.3390/pr9060953 https://www.mdpi.com/journal/processes Processes 2021, 9, 953 2 of 15 is known as the highest quality, containing total polyphenols in the range of 50 to 1000 mg/Kg, and acidity level below 0.8 g/100 g [11]. Processes 2021, 9, 953The functional potency of olive oil partly relies on the quantities of specific groups of 2 of 15 endogenous phenols [12] that are increased/decreased depending on the various pre/post- harvest activities. In this respect, there has been a growing research trend towards the exploration of the fateThe of functionalendogenous potency bioactive of olive molecules, oil partly relies particularly on the quantities polyphenols, of specific in groups of olive oil and its by-products,endogenous phenols through [12 various] that are agro-industrial increased/decreased steps. depending Together on with the various other pre/post- factors, the industrialharvest processing activities. and In thisstorage respect, conditions there has are been highly a growing influential research in the trend deg- towards the radation/formationexploration of polyphenols of the fate [11,13]. of endogenous The initia bioactivel point of molecules, the optimum particularly preservation polyphenols, in of endogenous phytonutrientsolive oil and its relies by-products, on a go throughod handling various of agro-industrial raw materials steps. that Together may be with other achievable throughfactors, a minimum/ca the industrialreful processing storage andtime. storage Ideally, conditions the storage are highly time influentialfor the har- in the degra- vested crops to bedation/formation processed for olive of polyphenols oil extraction [11,13 should]. The initialnot exceed point of48 the h [14]. optimum The preservationpro- of endogenous phytonutrients relies on a good handling of raw materials that may be achiev- longed storage of harvested olives may give rise to spoilage (oxidative and/or hydrolytic) able through a minimum/careful storage time. Ideally, the storage time for the harvested in the final productcrops [14]. to Together be processed with for the olive storage oil extraction factors, the should processing not exceed parameters 48 h [14]. Thedur- prolonged ing olive oil extraction,storage particularly of harvested olivesmalaxati mayon give temperature/time, rise to spoilage (oxidative plays crucial and/or roles hydrolytic) in in the the phenolic compositionfinal product of the [14 extracted]. Together oil, with which the storageis further factors, outlined the processing in more details parameters in during Section 2. olive oil extraction, particularly malaxation temperature/time, plays crucial roles in the Olive tree farming,phenolic in composition addition to ofthe the target/marketable extracted oil, which products, is further generates outlined ina range more details in of waste stream/by-productsSection2. (Figure 1) that is considered to be a good source of bio-func- tional molecules but theyOlive are tree currently farming, un in additionderused to for the limited/low-value target/marketable products,applications, generates sig- a range of waste stream/by-products (Figure1) that is considered to be a good source of bio-functional nifying the inefficiency of the processing system currently used for their extraction/recov- molecules but they are currently underused for limited/low-value applications, signifying ery. the inefficiency of the processing system currently used for their extraction/recovery. FigureFigure 1. Major 1. Major products products and and by-product by-productss produced produced from agro-industrialagro-industria activitiesl activities of oliveof olive fruits. fruits. Green technologies show potential for delivering optimal processing of olive oil Green technologiesproducts, show and sustainablepotential for valorization delivering of oliveoptimal mill processing by-products, of Although, olive oil there is a products, and sustainabletrend towards valorization the processing of olive developments mill by-products, in olive Although, oil industry there but, havingis a trend said that, the towards the processingproposed developments emerging methods in olive have oil not industry quite reached but, having the production said that, system. the pro- The existing posed emerging methodsmechanical have operations not quite commonly reached the used production in both olive system. oil production The existing and by-productme- re- chanical operationsutilization commonly often used represent in both (i) lessolive efficiency oil production of the processing and by-product of the main re-utili- products, and zation often represent(ii) low (i) value less applicationsefficiency of of the the processing processed by-products. of the main Thisproducts, review and describes (ii) the role of the processing on the fate of major phenolics in olive oil products and by-products low value applications of the processed by-products. This review describes the role of the and emphasizes the possibilities for a sustainable processing system that may enable processing on theachievement fate of major of phenolics the increased in olive recovery oil ofproducts health-promoting and by-products